I did a review on Topward’s 6603A linear power supply last time. In my opinion, Topward 6000 series power supplies are really well made and offer exceptional performance. One minor annoyance though, as I mentioned, is that the two cooling fans are constantly on. This makes the otherwise great piece of lab equipment rather noisy, even when the load is light.

My plan was to add a simple thermal controller that only switches on the fan when the temperature rises above a certain threshold. According to the service manual, the two 12V DC fans in the power supply are connected in series and powered by the 24V auxiliary power directly. So adding a temperature controller is a rather simple task.

The following diagram illustrates the heat sink layout of this power supply. It is essentially consisted of two separate heat sinks, one for each channel. The power transistors are mounted on a vertical fin that is perpendicular to the main heat dissipation surface. Due to the design, the temperature gradient can be quite high from the place where the TO3’s are mounted to the edge of the heat sink fins. So it is necessary to place the sensors as close to the power transistors as possible. Also, because the two channels of the power supply can be operated indecently we will need two temperature sensors, one for each channel. This arrangement can be seen in the diagram below.

Heat sink and sensor layout
Heat Sink and Sensor Layout

I could have designed a PWM controlled fan controller that automatically adjusts the fan speed depending on the heat sink temperature. But since for the majority of the time the power supply will be used well below its rated dissipation power, I actually prefer to only have to turn on the cooling fans when necessary and keep the on duration as short as possible. This way, the power supply can remain totally silent most of the time.

The circuit diagram below shows my simple design to accomplish this. A half LM393 dual comparator forms a Schmitt trigger which is used for each channel, and the control circuits are identical for both channels. The NTC thermistor I used is B57891M0103J000, which has a nominal resistance of 10K under room temperature (roughly 77 F) and 6.5K around 100 F. Using the resistor divider values shown in the circuit, the trip point (high threshold) is right around 100 F. The slight positive feedback introduced by R7 and R8 will give an approximately two degree window so the low transition temperature would be around 98 F. The output of the two comparators are then OR’d to drive the relay. Or course you don’t necessarily need to use a relay, you could just use a power transistor or MOSFET.

Dual-sensor Fan Controller
Dual-sensor Fan Controller

Here is a picture of the finished fan controller circuit board:

Fan Controller Board
Fan Controller Board

The relay can be inserted anywhere in the path of the fans connected in series. I chose to place the relay between the two fans so that I could use the exiting wiring. The only wiring change needed is the addition of the two wires that bring the 24V to the controller board as shown in the picture below (the original fan connections are marked using black marker).

Fan Controller Wiring
Fan Controller Wiring

There is plenty of room inside the Topward 6603A for me to place the controller board. I happen to have a case that snaps nicely between the transformer and the front panel.

Fan Controller Mounted
Fan Controller Mounted

Here are a couple of pictures showing the placement of the thermistors.

Thermistor Placement
Thermistor Placement
Thermistor Placement
Thermistor Placement

I am quite pleased with the result of this simple hack. Now my Topward 6603A is silent most of the time and the fans only come on briefly every so often when the power dissipation is high enough.

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41 Thoughts on “Adding a Fan Controller to Topward 6603A”

  • Can you send me a copy of the service manual? I am trying to fix my unit.
    Also it seems that your fan controller may be cooling the wrong part of the circuit. Yes the power transistors get hot, but they are mounted to those massive heat sinks which have free-flowing convection on them. I wonder if the placement of the fans was to keep some preamplifier circuit on the vertical circuit boards cool instead?


    • Sorry Ted, you will need to contact them directly. It’s their copyrighted material and I am not authorized to re-distribute. But if you have any specific questions I can certainly try to answer.

      The fans are for the heat sinks. They are placed on the top and sucks the air out of the case. The heatsinks while large, can’t dissipate the roughly 130W of power generated when the unit is at full power (the maximum voltage difference between the collector and emitter of the power transistor is around 22V on any taps). The heat sink actually gets quite hot if operated near maximum power for prolonged period of time.

    • Ted, did you have any luck locating a service manual/schematic for the Topward power supply? I to am looking for a service manual for the Topward 6303D.

      Thanks for your reply.

  • The fan control will prolong the life of the 2 fans

    I have a problem with the fans one seems to be getting about 9 volts and the other about 18 volts resulting in one cooling more than the other the powersupply is a topward 6303A

    • The two fans are put in series without any load balancing measures, which I think was a poor design decision. They could just used two 24V fans in parallel instead. You could swap out both fans by yourself I guess.

  • I have a 6303D that all of a sudden with no load applied at power on it does not display any volatge and the current when turn to max displays 0.009. This is on the right supply, the left supply works as normal. Trying to find a schematic with no luck. If you have the schematic any ideas where to start looking?

    Thank you for your help, greatly appreciate your reply.

    • Does it output anything (I know you said that the display was not changing)?

      The good news of this supply is that the master channel and the slave channel are largely identical so with one working channel you do have some reference points.

      I’d trace from the transformer side on and see where the problem started happening first to get some idea as to where to dig further.

  • Hi Kerry,
    Nice article. What tape did you use to fasten the thermistors onto the heatsinks? Is it a special heat resistance kind?

    • No, it was just the red electrical tape. Kapton tape would probably be better, but I didn’t have any that was wide enough back then. Anyway, it does the job and everything still works perfectly.

  • Good day, got a 6303ds but the right powersupply or channel does not go in protection when it exceeds current limit but it goes to it’s maximum current capability. Do you have an schematic diagram for this?

  • Hi Kerry,

    Thanks for this blog. I am repairing a 6603D power supply but having trouble obtaining a schematic. Are you able to able to help out?

  • Hi, have You any schematic of the 6603 , i’m fighting with a 6603D with a fixed high voltage output in both adjustable outputs.
    The output is 94 volts instead of be adjustable and the power transistors measured in circuit are OK

  • Can I please ask you to send me the schematic for the 6603D? I do not have anything on the display(s).


    – TIm

  • Thanks for the great article, just finished building and fitting the temperature controller in my 6303D power supply. It works great and makes the power supply much more user friendly. Next project, the transformer hum???

  • Hi!

    I’m looking for a service manual for the 6306D, but will settle for any version really since half my trouble is just getting the front panel disassembled without damaging the unit.

    Would you mind sending me a copy of a relevant manual?


  • I have a 6306A power supply and both outputs are measuring 51VDC the 5vdc is ok could you point me in the right area to look for the problem.

  • hmmm, interesting reading this column, I have a 6603D-10 that is blowing the input fuse on power up. I will be going through the likely suspects but if anyone has some lessons learnt your advise is much appreciated. Cheers, Rich

  • Hi, i found this discussion from google and it seems you have the schematic of a 6303d, altough i have a 6302d that needs repair (one channel is dead) i may be interested in it if you can send it email, regards, Pat

  • Good Day. I am Adrian Alexander From Advanced Digital Devices in South Africa.
    If possible, can you please email a copy of the schematics that you have for the 6603 Series power supplies.
    We have two units that have been shelved for almost 3 years after suffering severe burn damage on the pcbs.
    We are thus unable to make out what the part values are.
    Thank you. Regards, Adrian Alexander.

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